Magnetic fluid (MF) shows an increased saturation magnetization when exposed to a magnetic field. In order to improve the quality of servo-valve products by depressing and overcoming the self-excited high-frequency pressure oscillations and noise appearing in pilot stage of a hydraulic servo-valve, application of magnetic fluid in a hydraulic servo-valve is developed in this paper. Large damping can be introduced by the magnetic fluids into the servo-valve if magnetic fluids are filled into the working gaps of the hydraulic servo-valve torque motor. After the construction and working principle of a hydraulic servo-valve with magnetic fluids are introduced, the high-frequency pressure oscillation signals are tested and recorded when magnetic fluids are applied or not in the servo-valve. Experimental results are compared and analyzed by using FFT analysis method. It is shown that pressure oscillations of the servo-valve are depressed when magnetic fluids are applied.